1. Field of the Invention
The present invention relates to a seawater desalination system for desalinating seawater by removing salinity from the seawater and an energy exchange chamber (pressure exchange chamber) of an energy recovery device which is preferably used in the seawater desalination system.
2. Description of the Related Art
Conventionally, as a system for desalinating seawater, there has been known a seawater desalination system in which seawater passes through a reverse-osmosis membrane-separation apparatus to remove salinity from the seawater. In the seawater desalination system, the intake seawater is processed to have certain water qualities by a pretreatment system, and the pretreated seawater is delivered into the reverse-osmosis membrane-separation apparatus under pressure by a high-pressure pump. Part of the high-pressure seawater in the reverse-osmosis membrane-separation apparatus passes through a reverse-osmosis membrane against the reverse-osmosis pressure and is desalinated, and fresh water (permeate or desalted water) is taken out from the reverse-osmosis membrane-separation apparatus. The remaining seawater is discharged in a concentrated state of a high salt content as a concentrated seawater (reject or brine) from the reverse-osmosis membrane-separation apparatus. The largest operational cost in the seawater desalination system is energy cost for pressurizing the pretreated seawater up to such a pressure as to overcome the osmotic pressure, i.e. up to the reverse-osmosis pressure. That is, the operational cost of the seawater desalination system is greatly affected by pressurizing energy of the seawater by the high-pressure pump.
Specifically, more than half of the electric expenses as the highest cost in the seawater desalination system are consumed to operate the high-pressure pump for pressurizing the seawater. Therefore, pressure energy possessed by the high-pressure concentrated seawater (reject) with the high salt content which has been discharged from the reverse-osmosis membrane-separation apparatus is utilized for pressurizing part of the seawater. Therefore, as a means for utilizing the pressure energy of the concentrated seawater discharged from the reverse-osmosis membrane-separation apparatus to pressurize part of the seawater, there has been utilized an energy exchange chamber in which an interior of a cylinder is separated into two spaces by a piston arranged to be movable in the cylinder, a concentrated seawater port is provided in one of the two separated spaces to introduce and discharge the concentrated seawater, and a seawater port is provided in the other of the two separated spaces to introduce and discharge the seawater.
FIG. 19 is a schematic view showing a configuration example of a conventional seawater desalination system. As shown in FIG. 19, seawater pumped into the seawater desalination system by an intake pump (not shown) is processed to have certain water qualities by a pretreatment system, and then the pretreated seawater is delivered via a seawater supply line 1 into a high-pressure pump 2 that is driven by a motor M. The seawater which has been pressurized by the high-pressure pump 2 is supplied via a discharge line 3 to a reverse-osmosis membrane-separation apparatus 4 having a reverse-osmosis membrane (RO membrane). The reverse-osmosis membrane-separation apparatus 4 separates the seawater into concentrated seawater with a high salt content and fresh water with a low salt content and obtains the fresh water from the seawater. At this time, the concentrated seawater with a high salt content is discharged from the reverse-osmosis membrane-separation apparatus 4, and the discharged concentrated seawater still has a high-pressure. A concentrated seawater line 5 for discharging the concentrated seawater from the reverse-osmosis membrane-separation apparatus 4 is connected via a control valve 6 to a concentrated seawater port P1 of an energy exchange chamber 10. A seawater supply line 1 for supplying the pretreated seawater having a low pressure is branched at an upstream side of the high-pressure pump 2 and is connected via a valve 7 to a seawater port P2 of the energy exchange chamber 10. The energy exchange chamber 10 has a piston 12 therein, and the piston 12 is arranged to be movable in the energy exchange chamber 10.
The seawater pressurized by utilizing a pressure of the concentrated seawater in the energy exchange chamber 10 is supplied via the valve 7 to a booster pump 8. Then, the seawater is further pressurized by the booster pump 8 so that the seawater has the same pressure level as the discharge line 3 of the high-pressure pump 2, and the pressurized seawater merges via a valve 9 into the discharge line 3 of the high-pressure pump 2 and is then supplied to the reverse-osmosis membrane-separation apparatus 4.
In the above-described conventional energy exchange chamber, the piston in the energy exchange chamber is brought into sliding contact with the inner wall of the chamber, and thus the sliding member of the piston is required to be periodically replaced due to wear of the sliding member. Further, the inner diameter of the long chamber is required to be machined with high accuracy so as to fit with the outer shape of the piston, and thus machining cost is very expensive.
Therefore, the applicants of the present invention have proposed an energy exchange chamber having no piston in which a cylindrical and elongated chamber is used as a pressure exchange chamber and a plurality of partitioned fluid passages is provided in the chamber to directly pressurize the seawater with the high-pressure concentrated seawater which is discharged from the reverse-osmosis membrane (RO membrane) in Japanese Patent Publication No. 2010-284642.
In the energy exchange chamber disclosed in Japanese Patent Publication No. 2010-284642, in the case where the cylindrical and elongated chamber is installed horizontally, pressure is transmitted from the high-pressure concentrated seawater to the low-pressure seawater while the concentrated seawater and the seawater are separated into right and left parts in a horizontal direction and mixing of the concentrated seawater and the seawater is suppressed at a boundary where the two fluids are brought into contact with each other.
The present inventors have analyzed the energy exchange chamber in which an interface between the concentrated seawater and the seawater moves in the interior of the chamber by a pressure balance between the concentrated seawater and the seawater by a computer simulation that takes into consideration a difference in specific gravity of the concentrated seawater and the seawater, as disclosed in Japanese Patent Publication No. 2010-284642. As a result, the present inventors have learned that when the longitudinal direction of the chamber is placed horizontally, in the case where there is a difference in specific gravity, a problem arises.
Based on the above-described knowledge, the present inventors have conceived a means for separating the concentrated seawater and the seawater while mixing of the concentrated seawater and the seawater is suppressed even in the case where the longitudinal direction of the chamber is placed horizontally, and have made the present invention.
It is therefore an object of the present invention to provide an energy exchange chamber which can transmit a pressure from high-pressure concentrated seawater to seawater while the seawater and the concentrated seawater are separated into upper and lower and mixing of the seawater and the concentrated seawater is suppressed at a boundary where the two fluids are brought into contact with each other by supplying and discharging the concentrated water from a lower part of the chamber and by supplying and discharging the seawater from an upper part of the chamber.
Another object of the present invention is to provide a seawater desalination system having the above energy exchange chamber.